1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to chemical-mechanical polishing of semiconductor wafers using a linear polisher.
2. Description of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a base substrate to form the necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of a layer must be achieved in order to pattern and form various components and interconnects. Chemical mechanical polishing (CMP) is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. Other examples of CMP include flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to polish the exposed surface of the wafer (or a layer formed on the wafer), when subjected to a force pressing the wafer to the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface.
One technique for obtaining a more uniform chemical mechanical polishing rate is to utilize a linear polisher. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the global planarity is improved over CMP tools using rotating pads. One such example of a linear polisher is described in a pending application titled "Linear Polisher And Method For Semiconductor Wafer Planarization;" Ser. No. 08/287,658; filed Aug. 9, 1994. Unlike the hardened table top of a rotating polisher, linear polishers are capable of using flexible belts with separate pads disposed on the belts. This flexibility allows the belt to flex and change the pad pressure being exerted on the wafer.
A linear polishing tool generally has two separate consumables, a pad and a belt. The life span of a pad is short due to its use as the contact surface for polishing a semiconductor wafer and the need for conditioning the pad's surface during or between each polishing run. Although not replaced with the frequency of the pad, the belt also needs periodic replacement resulting from several causes including wear from the high operating speeds of the polisher, the heavy loads exerted on the belt during the polishing, and deformation or kinks due to accidents when replacing the polishing pads. The prior practice is to use separate polishing pads attached to stainless steel belts with an adhesive.
There are several disadvantages to using separate pads and belts with linear polishing tools. One disadvantage is that changing pads and or belts is both time consuming and costly. The mere act of replacing a pad and or a belt incurs a significant amount of time for labor. It typically takes about 15 to 20 minutes to install new pad strips on a belt, while the removal process of the old pad strips typically takes about 15 to 20 minutes. The cost associated with replacing belts and pads lies in the downtime associated with the their replacement. In the semiconductor industry, as with many industries, time is money. A linear polishing tool generally polishes one wafer every 2 to 3 minutes. Each additional or unnecessary minute spent replacing a pad and or a belt is lost revenue.
A pad (on a belt) generally consists of one or more strips of pad material with each strip being approximately equal to the belt width. One current example of a pad strip has a width of about 12 to 14 inches and a length of about 36 inches. The pad strips are put on the belt one at a time and must be carefully aligned to the belt and to each other. A very strong adhesive attaches the pad strips to the belt in such a way as to minimize and avoid the formation of air bubbles, which causes the pad strips to eventually separate from the belt.
When a pad wears out, it is necessary to replace all of the pad strips. The strips are removed from the belt by physically pulling or ripping them off of the belt. After removing the strips, it is necessary to remove the old adhesive from the belt. Removing the old adhesive usually requires using an organic solvent such as acetone or isopropyl alcohol. Great care is necessary during the removal process so as not to damage the belt since the belt by itself is typically only 0.02 inches thick.
Another disadvantage of the prior practice is the presence of one or more "seams" in the contact or polishing surface. A steel belt invariably has a noticeable welding seam that propagates through the pad to the polishing surface of the pad. The typical practice in manufacturing the belt is to take a rectangular piece of stainless steel and weld the ends together to form the stainless steel belt. The weld is then ground to smooth out the welded surface. Even with grinding the seam, there will still be some type of irregularity on the surface of the steel belt. After attaching the pad strips to the belt, this irregularity usually propagates through the pad so that the polishing surface of the pad will also have some irregularity or unevenness. Additional seams or irregularities on the polishing surface of the pad are produced when securing the pads to the belt. As previously noted, the typical practice is for the pads to be in rectangular strips before attachment to the belt. Another seam or some type of unevenness in the outer surface of the pad appears at the joinder of the two ends of the pad. Due to the small geometries required in semiconductor devices, any irregularities, unevenness, or seams on the pad's polishing surface will produce an uneven planarization on the surface of the semiconductor device.
The present invention describes an integrated pad and belt for polishing a surface such as glass or a semiconductor wafer. The integration of the pad with the belt reduces the down time of the linear polisher because there is only one piece to replace as opposed to the two pieces with the current practice. The manufacture of the integrated pad and belt allows a belt to be constructed without a noticeable welding seam, which reduces unevenness or irregularities on the polishing surface of the pad. Further, the integrating of the pad with the belt produces a seamless polishing surface, which further reduces the unevenness of the polishing surface of the pad. Still further, an integrated pad and belt eliminates trapped air bubbles between separate pads and belts resulting from replacing the pads. The present invention, therefore, reduces the number of defects by promoting a better polishing uniformity, and improves reliability by reducing the number of steps required to replace pads and belts, while at the same time, decreasing the down time of the linear polishing tool.